1. Technical Field
The present invention relates to ink sets including at least cyan, magenta, and yellow ink compositions and ink-jet recording methods for ejecting ink compositions onto recording media to perform printing.
2. Related Art
Ink-jet recording is a printing method of ejecting ink droplets onto recording media, such as paper, to perform printing. Inks are typically prepared by dissolving various water-soluble dyes in water or water and a water-soluble organic solvent. It is generally pointed out that an image formed using an ink containing a water-soluble dye has low water resistance and light fastness.
In contrast, inks prepared by dispersing pigments in aqueous media have high water resistance and light fastness. For example, aqueous pigment inks prepared by dispersing pigments with surfactants or polymer dispersants have been proposed. These inks, however, may exhibit sharply increased viscosity as the colorant content is increased to increase the print density of a recorded material. In addition, excessive surfactant or polymer dispersant is needed to stably disperse the pigments in the inks. This may foam the inks and degrade the defoaming properties thereof, thus impairing the print stability.
To solve these problems, disclosed are self-dispersible pigment dispersions, which contain pigments having a certain amount or more of surface active hydrogen or salt thereof introduced to the surface thereof so that they can be dispersed in an aqueous solvent by themselves without the use of a dispersant such as a surfactant or a polymer dispersant.
According to JP-A-10-237349, JP-A-8-3498, and WO01/94476, an ink containing a pigment requiring no dispersant, namely, a self-dispersible pigment, as a colorant forms an image with a higher optical density (OD), is easy to handle because the viscosity thereof can be easily adjusted to an appropriate range, and eliminates the need for taking into account compatibility between dispersants and various additional solvents. The surface of the pigment has hydrophilic functional groups, such as carbonyl, carboxyl, hydroxyl, sulfo, or ammonium groups, attached thereto directly or indirectly with other functional groups, such as alkyl or aryl groups, therebetween.
As an ink set including inks containing self-dispersible pigments, for example, JP-A-2002-121434 discloses an aqueous color ink set for ink-jet recording including a black ink and, as color inks, at least cyan, magenta, and yellow inks. The black ink and the color inks contain pigments self-dispersible in water. This publication, however, does not disclose an ink set in which only the yellow ink composition contains a pigment coated with a water-insoluble polymer.
In addition, JP-A-2009-256605 discloses an ink set including a black ink composition containing a self-dispersible pigment having hydrophilic groups on the surface thereof and color ink compositions containing self-dispersible pigments having hydrophilic groups on the surface thereof with phenyl groups therebetween. This publication, however, does not disclose an ink set in which only the yellow ink composition contains a pigment coated with a water-insoluble polymer.
In addition, JP-A-2007-51176 discloses an ink set including a black ink composition containing a self-dispersible pigment and color ink compositions containing organic pigments coated with a water-insoluble polymer. This publication, however, does not disclose an ink set which includes at least cyan, magenta, and yellow ink compositions and in which only the yellow ink composition contains a pigment coated with a water-insoluble polymer.
However, self-dispersible pigments generally have a problem in that an image formed using an ink containing a self-dispersible pigment as a colorant has low fusibility. The image is smudged when rubbed with, for example, a finger, a marker, or paper; that is, the recorded material may have insufficient smear resistance. Similarly, an image formed using an ink containing a pigment coated with a water-insoluble polymer as a colorant may have insufficient fusibility and smear resistance, particularly if the ink composition has a high pigment concentration. To improve the fusibility of the pigments on recording media, the addition of a resin having a binder effect on recording media to the ink composition has been proposed.
JP-A-2001-81366 and 2002-80761 propose ink compositions containing resin particles, specifically, a recording liquid containing a colorant, fine resin particles, and water and an ink printer recording liquid containing an aqueous pigment dispersion and fine resin particles.
In addition, JP-A-2009-256606 discloses an ink set including a black ink composition containing a self-dispersible pigment having hydrophilic groups on the surface thereof and color ink compositions containing self-dispersible pigments having hydrophilic groups on the surface thereof with phenyl groups therebetween, each further containing a resin emulsion. This publication, however, does not disclose an ink set in which only the yellow ink composition contains a pigment coated with a water-insoluble polymer.